1. Field of the Invention
This invention relates to a circuit interrupting switch for use at relatively high voltages and more specifically to an SF.sub.6 puffer interrupter switch that permits the operator to view the interrupting contacts from the outside of the switch housing.
2. Description of the Prior Art
Circuit interrupting switches, such as SF.sub.6 puffer interrupter switches, are generally old and well-known in the art. An example of such a switch is disclosed in U.S. Pat. No. 3,947,650, assigned to the assignee of the present invention. In the interrupter switch of the '650 patent, a conductive switch housing is provided with SF.sub.6 gas or another dielectric gas. At least one pair of insulated feedthrough bushings are provided that are hermetically sealed to the housing. Each bushing includes a conductive member, a portion of which projects internally in the housing and a portion which projects externally of the housing. The external portions of the conductive members of the bushings are serially electrically connected to an external electrical circuit to provide the interrupting capability. The internal portions of the conductive members are coupled to an interrupting assembly located within the housing that includes a pair of interrupting contacts housed within a puffer tube, in which a stream of gas, such as SF.sub.6 gas, is directed at the interrupting contacts as they begin to separate to extinguish the resulting arc.
The pressure of the SF.sub.6 gas within the housing is generally greater than atmospheric pressure. In some known SF.sub.6 puffer interrupters, the pressure of the SF.sub.6 gas within the puffer tube is maintained at an even greater pressure than the pressure of the SF.sub.6 gas within the housing to prevent liquification of the SF.sub.6 gas at relatively low temperatures. Since the interrupter switch housing is pressurized, it is imperative that the housing be hermetically sealed to prevent the SF.sub.6 from leaking, which could result in severe damage due to a resulting flashover within the interrupter switch assembly.
Puffer interrupter switches have been used, for example, to provide bus isolation for a unit substation bus from an incoming line feed during bus maintenance. However, prior to performing maintenance on the bus, it is necessary for the operator to verify that the interrupting contacts within the puffer tube are indeed open. In prior art air interrupter switches, visible break switch windows are provided on the housing to enable the operator to view the position of the interrupting contacts. However, leakage of an air interrupter switch will not result in damage to the switch since the air itself is the dielectric. Heretofore, such viewing windows have generally not been used on SF.sub.6 interrupting switches because of the possibility of leakage of the SF.sub.6 gas which may result in equipment damage.
Indirect methods thus have been utilized to determine the position of the interrupting contacts in an SF.sub.6 puffer interrupter switch. One such indirect method is to examine the incoming line and bus instrumentation, such as bus voltmeters and bus ammeters, external to the interrupter. For example, if an incoming line voltmeter indicated 38 kv and the bus voltmeter indicated zero volts. This would be an indirect indication that the interrupter switch was open. An operator maY also examine a bus ammeter. However, such a method of determining the position of the interrupter switch contacts is only an indirect method which is subject to errors resulting from, for example, an operator reading the wrong voltmeter or a faulty voltmeter. This method also requires external bus instrumentation which may not be otherwise practical.
Colored status flags have been provided on some interrupting devices, such as interrupting switches and circuit breakers, that are coupled to the main interrupting contacts. A red flag is used to indicate that the contacts are closed while a green flag is used to indicate that the contacts are open. An example of status flags provided on a high voltage fuse is disclosed in U.S. Pat. No. 4,186,365. However, status flags only provide an indirect indication and are subject to malfunction.